5'-Methylthioadenosine (MTA) is enzymatically synthesized by at least five separate pathways including polyamine biosynthesis. Although the cellular functions of MTA are unknown, recent reports indicate that exogenous MTA can dramatically inhibit cellular biosynthetic processes. Of particular interest are our recent findings which indicate the inhibitory effects of exogenous MTA on DNA and protein synthesis in activated lymphocytes. The overall objectives of this proposal are: a) to document further the importance of MTA metabolism in cell proliferation and b) to study the mechanisms by which MTA can inhibit cell activation. Specific objectives to be completed include: a) Describing the effects of MTA and its analogues on activated cells from a range of biological systems (including stimulated lymphocytes, continuous cell lines from various tissue sources, and tumor tissue). b) Developing sensitive methods for measuring intracellular MTA levels and levels of the MTA phosphorylase. c) Monitoring levels of MTA, its precursors and breakdown products in quiescent and actively metabolizing tissues. d) Monitoring activity of enzymes involved in MTA biosynthesis and degradation in quiescent and activated tissue. e) Analyzing the mechanisms by which MTA inhibits cell metabolism. The production and degradation of MTA in activated cells, coupled with its kn wn inhibitory effects on biosynthesis implicate MTA as having important regulatory potential. Understanding the regulatory mechanisms of MTA may lead to more effective control of neoplastic cell growth and immune responsiveness.